While heterosexual transmission of human immunodeficiency virus (HIV) is accepted, the efficiency of transmission by type of sexual activity or other factors and the frequency of sexual contact between infected and uninfected heterosexuals are unknown. Also unknown are the prevalence of infection among prostitutes. The lack of such information makes it difficult to predict the future rate of growth of the AIDS epidemic or to plan intervention programs. The primary aims of this project thus are (1) to monitor the prevalence of infection with HIV among heterosexually active males and females and among female prostitutes; and (2) to investigate risk/protective factors associated with HIV infection in heterosexual men and women. Risk/protective factors to be studied include numbers of sexual partners, types of sexual activities, sexual contact with members of AIDS risk groups, protective effects of contraceptive devices, sexual contact with prostitutes and vaginal or penile lesions. The epidemiology of infection with human T-cell leukemia virus (HTLV-1) is less well-understood than that of HIV, despite the recent finding of a substantial rate of HTLV-I seropositivity among New York City intravenous drug users. Therefore, a secondary aim of the present proposal is to determine the seroprevalence of HTLV-I antibodies among sexually active persons and to investigate the variation in seropositivity by demographic characteristics and behavioral patterns. Subjects will be interviewed and donate samples of blood for HIV and HTLV-1 analyses. Statistical analyses will determine the variation in HIV seropositivity by age, sex, race, and AIDS risk groups of sexual partners. HIV seropositive subjects will be matched on age, race and sex to seronegative subjects using a 4:1 matching ratio to conduct case-control analyses. Logistic regression will be used to separately investigate risks in males and females. The proposed sample sizes of about 2,100 males and 2,100 females should provide a power of 80% (at a 1-tailed alpha significance level of 0.05) to detect risk ratios of 3.0 or greater in each sex group. Data on HTLV-1 will be used to assess the seroprevalence and potential threat from this virus alone and in concert with HIV infection.